Structure, Foreland Basin Evolution and Kinematics of the Montana Disturbed Belt, Northwestern U.S.A.

Facundo Fuentes, Peter DeCelles, and Kurt Constenius
University of Arizona, Tucson, AZ

The structure of the northwestern Montana fold-thrust belt (Montana Disturbed Belt) is dominated by a several-kilometer thick megathrust sheet of Precambrian Belt Supergroup and Paleozoic strata brought to the surface by the Lewis thrust and related thrusts. A belt of closely spaced imbricate thrusts carrying Paleozoic and Mesozoic rocks characterizes the structure in the footwall of the megathrust sheet. A new regional balanced cross section indicates a total minimum shortening of ~125 km for this region.

Stratigraphic and provenance data indicate that at least by the beginning of the late Jurassic, a foreland basin system was established in the region. These first preserved foreland basin deposits accumulated in a backbulge depozone. The presence of palaeosols and a ~20 Myr regional unconformity bounding the top of the Morrison Formation suggest the cratonward migration of a forebulge. By late Neocomian time a foredeep was established in northwestern Montana, and developed until the Early to mid(?)-Paleocene, accumulating ~3,000 m of foreland basin strata. No wedge-top depozone exists in the area.

The presence of a mid(?) to late Jurassic foreland basin implies that a fold-thrust belt was active at that time. However, cross-cutting relationships, thermochronology and geochronology suggest that most of the shortening in the frontal part of the fold-thrust belt occurred during the mid-Campanian to Late Paleocene. This gives a shortening rate for the fold-thrust belt of 8.3 mm/y. From middle Eocene to early Miocene, the Cordillera collapsed, generating a number of extensional depocenters, which locally accumulated more than 3,000 m of sediments.